In a related-art air-conditioning apparatus, such as a multi-air-conditioning apparatus for a building, a refrigerant is circulated between an outdoor unit, functioning as a heat source unit, disposed outside a structure, for example, and an indoor unit disposed in an indoor space in the structure. The refrigerant transfers heat or removes heat to heat or cool air, thus heating or cooling a conditioned space with the heated or cooled air. As regards the refrigerant used in such an air-conditioning apparatus, for example, an HFC (hydrofluorocarbon) refrigerant is often used. An air-conditioning apparatus recently developed uses a natural refrigerant, such as carbon dioxide (CO2).
In an air-conditioning apparatus called a chiller, cooling energy or heating energy is produced in a heat source unit disposed outside a structure. Water, antifreeze, or the like is heated or cooled by a heat exchanger disposed in an outdoor unit and it is conveyed to an indoor unit, such as a fan coil unit or a panel heater, to perform heating or cooling (refer to Patent Literature 1, for example).
An air-conditioning apparatus called a waste heat recovery chiller is constructed such that a heat source unit is connected to each indoor unit by four water pipes arranged therebetween and, for example, cooled water and heated water are simultaneously supplied so that cooling or heating can be freely selected in the indoor unit (refer to Patent Literature 2, for example).
Another air-conditioning apparatus recently developed is constructed such that a heat exchanger for a primary refrigerant and a secondary refrigerant is disposed near each indoor unit to convey the secondary refrigerant to the indoor unit (refer to Patent Literature 3, for example).
Another air-conditioning apparatus recently developed is constructed such that an outdoor unit is connected to each branching unit including a heat exchanger by two pipes to convey a secondary refrigerant to an indoor unit (refer to Patent Literature 4, for example).
Air-conditioning apparatuses, such as a multi-air-conditioning apparatus for a building, include an air-conditioning apparatus constructed such that a refrigerant is circulated from an outdoor unit to a relay unit and a heat medium, such as water, is circulated from the relay unit to each indoor unit to reduce conveyance power for the heat medium while circulating the heat medium, such as water, through the indoor unit (refer to Patent Literature 5, for example).
A system (for example, an air-conditioning and hot water supply combined system) recently developed is constructed such that a first refrigeration cycle, such as a multi-air-conditioning apparatus for a building, and a second refrigerant cycle allow heat exchange to produce hot water (refer to Patent Literature 6, for example).